There are kinds of goods comprising pumps (e.g. motor tire inflators and air compressors for vehicle horns) which are noisy but for which the sales market for such goods is so large and the operational requirements so simple that the cost thereof can be minimized by volume mass production. However, such pumps are inherently noisy and are not suitable for other purposes where noise is detrimental. Additionally, there are requirements for relatively small quantities of pump units for special applications requiring complex control systems, such as pump units for air pump apparatus for powering and controlling pneumatically operated equipment, e.g. lifting seats for disabled persons or dentists drills, but such specialized pump units are invariably relatively expensive, complex and massive.